Generally, plasma spraying includes spraying of molten or heat softened material onto a substrate to provide a coating on the substrate. Typically, material in the form of powder is injected into a very high temperature plasma flame where it is rapidly heated and accelerated to a high velocity. The hot material, directed toward the substrate, impacts a surface of the substrate and rapidly cools forming a coating thereon.
A plasma spray gun usually includes a cathode and anode both of which are water-cooled. The anode being formed in the shape of a nozzle of the spray gun. An inert gas, such as argon, is directed around the cathode and through the nozzle. A plasma flame is initiated by a high voltage discharge which causes localized ionization and a conductive path for an arc to form between the cathode and anode. Resistance heating from the arc causes the gas to reach extreme temperatures and ionize forming a plasma flame directed through the nozzle. A coating material injected into the plasma flame is rapidly heated and accelerated through the nozzle toward a substrate to be coated. The heated material impacts the substrate where it rapidly cools forming a coating thereon.
Generally, a plasma spray gun using an inert gas such as argon can produce a thermal plasma of very high temperatures, up to 20,000 degrees Centigrade. Typically, the electrical current used to produce the arc between the cathode and anode is approximately 500 Amperes or more. Due to the high temperatures involved with the above-described plasma spray guns, a water cooling system is also provided for cooling both the anode and cathode and associated parts of the spray gun.
The F4 plasma spray gun is widely used in the thermal spraying industry and has become one of the predominant plasma spray guns currently used. Referring to FIGS. 1 and 2, a prior art F4 plasma spray gun includes a rear section 9 for supporting an electrode 8 which provides the negative pole or cathode of the plasma gun. A center section 5 supports an anode holder 4 and includes a mount for attaching the spray gun to a manipulator. The anode holder 4 receives a water cooled nozzle 3 which forms the positive pole or anode for the electric arc. Referring to FIG. 2, the center section 5 of the F4 gun also includes a water inlet tube 10 attached to a coolant tube block (not shown).
Although not clearly shown in FIGS. 1 and 2, one of the drawbacks of the F4 gun is that major components of the spray gun, including the center and rear sections thereof, are made from multiple pieces of lightweight brass brazed together. Thus, if one component of the center or rear sections of the plasma gun becomes damaged or worn, the entire center section, or entire rear section of the gun must be replaced. Typically, these components are expensive pieces such that replacement thereof is costly and significantly adds to the operating costs of the F4 plasma gun.
The manufacturing process for the center section of the F4 gun requires the brazing of multiple parts, one to the other, followed by pressure testing of the brazed joints. Thus, each of a multiplicity of parts is first machined or otherwise formed prior to the assembly thereof via brazing. Following the assembly process the parts typically undergo a finishing process wherein the parts are machined to final specifications. Thereafter, the finished products are tested, i.e., the brazed joints of the center section require testing (typically pressure testing) to confirm the integrity of the joints and the resulting seal formed between the brazed together parts. Any parts that fail a pressure test are presumably scrapped or recycled, which further adds to the cost of the manufacturing process.
Typically, in conjunction with the maintenance of the of the F4 spray gun, the gun is taken apart and inspected on a regular basis. The nozzle and electrode as well as O-rings positioned between adjacent components are replaced periodically. If any portion of the center section is worn or otherwise damaged, the entire center section must be replaced. The center section of the F4 spray gun can also be damaged during disassembling and re-assembling of the spray gun, which also results in the replacement of the entire center section.
Another disadvantage of the center section of the F4 plasma spray gun is that the coolant tube at an outlet end thereof is inserted into an opening in a coolant tube block and fluidly sealed thereto via a brazed joint therebetween. This brazed joint is somewhat fragile and susceptible to damage and/or leaks caused by twisting of the coolant tube during the coupling or uncoupling of a coolant source line to the coolant tube. Again, if the coolant tube is damaged or a leak occurs, the entire center section must be replaced prior to operating the spray gun.
A further disadvantage of the center section of the F4 plasma spray gun is the cost of replacement thereof. Due to the complex manufacturing process described above, the cost of the center section is relatively expensive and adds considerably to the cost of operating the spray gun especially since the functional life of the center section is relatively short due in part to the unitary design thereof.
Based on the foregoing, it is the general object of the present invention to provide an improved modular anode support member for use with plasma spray guns including an F4 spray gun that improves upon, or overcomes the problems and drawbacks associated with the prior art.